In a conventional exhaust gas recirculation apparatus, an exhaust pipe is communicated with a suction port via external piping. A normally closed EGR valve arranged intermediately of the external piping is opened during a suction stroke by utilization of negative pressure in the suction port to thereby recirculate the exhaust gas through the external piping.
However, the above-mentioned conventional exhaust gas recirculation apparatus provides lean combustion during the suction stroke since the exhaust gas is always sucked into the combustion chamber during the suction stroke. Though satisfactory combustion may be attained in light-load operating ranges where the air is inherently excessive, unsastifactory combustion may be disadvantageously brought about in high-load operating ranges where the ratio of air to fuel is lower, which tends to produce sooty black smoke.
Furthermore, disadvantageously, a required installation space for the engine is increased because of the external piping with the EGR valve required. Consideration must be taken to thermal insulation and arrangement constraints of the external piping which will reach high temperatures due to the exhaust gas flowing through the external piping.
Moreover, there is a problem for example in an engine with a turbocharger that exhaust gas cannot be satisfactorily recirculated in operating ranges where the boost pressure (supercharging pressure in a suction pipe) is higher than the exhaust pressure.
The present invention was made in view of the above facts and has its object to provide an exhaust gas recirculation apparatus in which exhaust gas can be recirculated only in operating ranges where it is required, in which the exhaust gas can be recirculated without use of external piping and in which the exhaust gas can be satisfactorily recirculated even in operating ranges where the boost pressure is higher than the exhaust pressure for example in a case of an engine with a turbocharger.